Important requirements for materials used in boiling water nuclear reactor construction include low absorption for thermal neutrons, corrosion and stress-corrosion resistance and mechanical strength. Zirconium-base alloys sufficiently satisfy these requirements that they are widely used for such purposes, "Zircaloy-2" (containing about 1.5 percent tin, 0.15 percent iron, 0.1 percent chromium, 0.05 percent nickel and 0.1 percent oxygen) and "Zircaloy-4" (containing substantially no nickel and about 0.2 percent iron but otherwise similar to Zircaloy-2) being two of the important commercial alloys commonly finding such use. These alloys, however, are not nearly all that one would desire, particularly in respect to accelerated pustular corrosion which occurs under boiling water reactor normal operating conditions and results in spalling of thick oxides from channels and thickening of oxides on fuel rods. The spalling of oxide flakes leads in some instances to development of high radiation fields in locations where the flakes collect; in addition, the extra loss of metal thickness due to the accelerated oxidation process requires an undesirable increase in design allowances for corrosion.
Efforts heretofore to solve this particular problem have to our knowledge met with no success, although the general subject of corrosion of such alloys has long been of active interest to experts in the field. Thus, in U.S. Pat. No. 3,005,706, it is proposed that from 0.03 to 1.0 percent of beryllium be added to zirconium alloys intended for use in conventional boilers, boiling water reactors and similar apparatus to enhance corrosion resistance to high temperature water. Similarly, in U.S. Pat. Nos. 3,261,682 and 3,150,972, cerium and/or yttrium and calcium, respectively, are proposed as zirconium alloy additions in like proportions of the same purpose. Accounts and reports of the long-term results of such compositional changes are sparse, however, and commercial zirconium alloys do not include these additional constituents.